In semiconductor elements including a silicon carbide semiconductor, as an element structure, an epitaxially-grown layer grown on a low-resistance substrate is frequently used as an operating layer. In power semiconductor elements, an epitaxially-grown layer functions as a voltage sustaining layer, and the epitaxially-grown layer is normally formed as a single layer (for example, see Patent Document 1). In many cases, the epitaxially-grown layer has a thickness of 3 to 100 μm or more depending on a voltage for operation, and has a doping concentration on the order of 1016 cm−3 at most or on the order of 1015 cm−3 in many cases. On the contrary, a dopant of about 1019 cm−3 is doped in a low-resistance crystal that becomes a substrate in many cases. Accordingly, the doping concentration differs considerably between the epitaxially-grown layer (voltage sustaining layer) and the substrate, which leads to a lattice constant difference therebetween. In a case where the epitaxially-grown layer has a larger thickness, the crystal quality of the epitaxially-grown layer degrades due to a lattice constant difference, that is, the introduction of a crystal defect accompanying lattice misfit. As a result, the carrier mobility decreases, leading to a problem of an increase in element resistance.
Therefore, as to the silicon carbide crystal having a (11-20) plane, it is disclosed that a buffer layer having a doping concentration of 2×1015 to 3×1019 cm−3 and a layer thickness of 0.3 to 15 μm is provided between the substrate and the epitaxially-grown layer for mitigating the effects on the crystal quality due to a lattice constant difference, and it is described that a single layer film, stepwise gradient structure or continuous gradient structure in the range of the above-mentioned doping concentration and layer thickness is provided (for example, see Patent Document 2).
As to silicon carbide crystals having a (0001) plane and a (000-1) plane, it is described that as a buffer layer provided between a substrate and an epitaxially-grown layer, a plurality of layers having a doping concentration of approximately 1/10 to ½ of the doping concentration of a substrate serving as a ground are laminated to provide stepwise gradient film whose doping concentration changes stepwise for preventing the introduction of basal-plane dislocation into the epitaxially-grown layer (for example, see Patent Document 3).